In reply to JJL:
Temperature. El Cap is a bit of a pain for really hard free climbing as it's in the sun a lot of the time. It may be January, but in the sun it's still pretty warm - far too warm to do really sharp crimpy technical vertical wall climbing without tearing your skin to shreds. Within (pretty ridiculous) limits, for that kind of climbing, the colder the better. So even out of the sun, it seems they are waiting to after dark for optimal conditions (not particularly unusual in the bouldering world.) Arguably, one of the main reasons that this ascent is now finally happening, is that they've figured out that you can free climb on El Cap in January without being taken out by ice avalanches...
(They started off trying it in October - the normal time of year for free climbing in the autumn (which works fine on stuff up to around Fr8b, especially on cracks etc), and each season they've tried it progressively later and later in the year until now they've finally realised that it's o.k to climb on there in mid winter - as long as you get a decent weather window and there isn't too much melting snow above your head!)
Post edited at 15:55